Postseismic deformation due to the Mw 6.0 2004 Parkfield earthquake: Stress‐driven creep on a fault with spatially variable rate‐and‐state friction parameters

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Abstract
We investigate the coseismic and postseismic deformation due to the Mw 6.0 2004 Parkfield, California, earthquake. We produce coseismic and postseismic slip models by inverting data from an array of 14 continuous GPS stations from the SCIGN network. Kinematic inversions of postseismic GPS data over a time period of 3 years show that afterslip occurred in areas of low seismicity and low coseismic slip, predominantly at a depth of ∼5 km. Inversions suggest that coseismic stress increases were relaxed by predominantly aseismic afterslip on a fault plane. The kinetics of afterslip is consistent with a velocity‐strengthening friction generalized to include the case of infinitesimal velocities. We performed simulations of stress‐driven creep using a numerical model that evaluates the time‐dependent deformation due to coseismic stress changes in a viscoelastoplastic half‐space. Starting with a coseismic slip distribution, we compute the time‐dependent evolution of afterslip on a fault plane and the associated displacements at the GPS stations. Data are best explained by a rate‐strengthening model with frictional parameter (a − b) = 7 × 10−3, at a high end of values observed in laboratory experiments. We also find that the geodetic moment due to creep is a factor of 100 greater than the cumulative seismic moment of aftershocks. The rate of aftershocks in the top 10 km of the seismogenic zone mirrors the kinetics of afterslip, suggesting that postearthquake seismicity is governed by loading from the nearby aseismic creep. The San Andreas fault around Parkfield is deduced to have large along‐strike variations in rate‐and‐state frictional properties. Velocity strengthening areas may be responsible for the separation of the coseismic slip in two distinct asperities and for the ongoing aseismic creep occurring between the velocity‐weakening patches after the 2004 rupture.

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